1. Field of the Invention
The present invention relates to a high-power light source including at least one high-power laser diode module.
2. Discussion of the Background
Generally, a laser diode module is used as a signal light source for optical fiber communication, especially for a main line system or a CATV, or as an excitation light source for optical fiber amplifier. Such a laser diode module includes a peltier device in order to attain high power and stable operation. The laser diode module further includes a laser diode chip, a photo diode chip, optical components such as lenses, and electrical components such as a thermistor device or registers, which are supported by a metal substrate that is mounted on the peltier device.
The above mentioned peltier device is an electrocaloric semiconductor. When a direct current flows through the peltier device, the heat is transferred toward the direction of the electric current flow, thereby causing temperature difference between one end of the electrocaloric semiconductor and the other end thereof. A cooling system utilizing the peltier device uses the lower temperature side for cooling and the higher temperature side for heat radiation thereby making use of the above mentioned temperature difference.
The laser diode module detects the temperature of the laser diode chip by using the thermistor device bonded in the vicinity of the chip. Thus provided is a structure for cooling the entire metal substrate on which the laser diode chip is placed, and for maintaining the laser diode chip at a fixed temperature, by activating the peltier device by feeding back the detected temperature.
FIG. 4 shows a schematic sectional view of a conventional laser diode module. The laser diode module includes a metal substrate 110a on which is mounted a mount 113 supporting a laser diode chip 111 and a heat sink 112, a chip carrier 115 supporting a monitoring photo diode chip 114, and a lens holder 116. The metal substrate 110a further has resistors, inductors, circuit substrates, etc. that are bonded thereto, which are not depicted in FIG. 4. The metal substrate 110a is bonded to a peltier device 117. The peltier device is fixed on a package radiator board 118 with metal solder. Here, ceramic substrates 119A, 119B are placed on the upper and lower side of the peltier device 117.
FIG. 5 is a cross-sectional view of the laser diode module taken along line V—V in FIG. 4. As is shown in FIG. 5, in the main portion of the laser diode module, the thermistor 121 and the laser diode chip 111 are mounted on the heat sink 112. Soft solder 122 is employed as metal solder for joining the peltier device 117 and the metal substrate 110a, in order to relieve stresses caused by the difference in thermal expansion between the materials used to construct the peltier device 117 and the materials used to construct the metal substrate 110a. 
The above mentioned metal substrate 110a is generally made of a single material, such as cuprotungsten (CuW: weight ratio of copper can be 10 to 30 percent). A low temperature soft solder, such as indium-tin (InSn), has been employed for bonding between the metal substrate 110a and the peltier device 117, in order to relieve stresses caused by the difference in thermal expansion between the materials used to construct the peltier device 117 and the materials used to construct the metal substrate 110a. 
However, in recent years, the requirement for cooling ability and the temperature environment reliability (i.e., the ability to maintain regular function even if the temperature changes) of the laser diode module is becoming more challenging, as the power of the laser diode module is increased, thereby increasing the amount of heat generated by the laser diode module.
Possible solutions to the above problem include enlarging the size of the peltier device or employing a material with high thermal conductivity in order to enhance the cooling ability. However, such changes in configuration cause an increase in temperature stress to the metal substrate mounted above the peltier device, because of the shortening of the temperature controlling time (i.e., the time needed to reach a target temperature) entailed by the enhancement of the cooling ability of the peltier device. Therefore, there exists a problem in that the influence of the difference in thermal expansion coefficients between the peltier device and the metal substrate is enhanced or increased, thereby causing cracking and peeling off to occur due to the vibration of the soft solder bonding them. Accordingly, since the phenomenon of solder creep (which is typical for soft solder) becomes significant in such a configuration, it becomes necessary to employ a low temperature hard solder, such as a bismuth-tin (BiSn), as solder for bonding the peltier device and the metal substrate.
In order to resolve the above-mentioned problem, a semiconductor laser module having a metal substrate consisting of two kinds of metals is described in Japanese Patent Application Laid-open No. Hei 10-200208. The semiconductor laser module is illustrated in FIGS. 6A and 6B. As is shown in FIG. 6A, the semiconductor laser module is fabricated by bonding a metal substrate 210 and a peltier device 207 with hard solder 212. The substrate 210 has mounted thereon an LD chip 201, a thermistor 211 for keeping the temperature of the LD chip 201 constant, a heat sink 202, and a sub-mount 203. The peltier device is provided with ceramic substrates 209A, 209B at its top and bottom, respectively.
The metal substrate 210 is bonded to the upper surface of the peltier device in a direction perpendicular to a direction of heat flow from the LD chip 201 to the peltier device 207. The metal substrate 210 is constructed such that a first metal member 213 is positioned at a central portion of the substrate, which includes a portion underneath the LD chip 201. The metal substrate 210 also includes a second metal member 214 is placed on the sides surrounding the first metal member 213, as depicted in FIG. 6B. Further, the first metal member 213 of the metal substrate 210 is composed of a metal material with high thermal conductivity and the second metal member 214 is composed of a material with lower thermal conductivity compared to that of the first metal member 213. The metal substrate 210 is expected to reduce the thermal expansion of the entire metal substrate, to improve the thermal conductance, and to improve the cooling ability, thereby increasing the reliability of the peltier device.
The laser diode module depicted in FIGS. 6A and 6B is intended to improve the cooling ability of the peltier device and heighten the reliability of the peltier device. However, if the output power of the laser diode module is increased and if such laser diode modules are used in large numbers in highly dense placement, then there exists a problem that the function of the laser diode module is damaged. Under such conditions it becomes impossible to manage the heat generated by the enhancement of the output power of the laser diode modules and the dense placement of the laser diode modules merely by increasing the thermal conductivity of the metal substrate placed between the chip and the peltier device, and by decreasing the difference in thermal expansion coefficients between them.
As each laser diode module is small in size, and as each module act as a high density heat element, it is difficult to release the heat of the laser diode modules when the laser diode modules are used as a light source for optical excitation or as a light source for optical signals where it is necessary to mount a plurality of laser diode modules. As high power is required for the light source for optical excitation or the light source for optical signals, and as the cooling ability of the peltier device has reached an upper limit of efficiency in the conventional laser diode module configurations, semiconductor laser devices utilizing conventional modules are forced to operate in an inefficient manner below their full abilities.
In addition, there is a desire in the market to maintain the electric power consumption in operating the peltier device and the semiconductor device at or below current levels, while the optical output is improved.
Accordingly, there is a need for an improved high-power light source that overcomes the problems identified above.